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Full text of "Practical Organic Chemistry"

134               PRACTICAL ORGANIC CHEMISTRY

quantity (about 20 grams) of crude leucine in the form of a brown
crystalline crust separates, and is collected on a filter and
dried on a porous plate. It is converted into the ester
hydrochloricle as follows: the dry material is dissolved in
120 c.c. absolute alcohol and saturated with hydrogen chloride
(p. 93). The alcohol is removed- by distilling under reduced
pressure at a temperature not exceeding 40 in the apparatus
shown in Fig. 66 (p. 94). The same quantity of alcohol is added,
saturated with hydrogen chloride, and removed as before. The
residue, which consists of the ester hydrochloride of leucine and
small quantities of other amino-acids, is converted into the free
ester in the following way : it is dissolved in about one-quarter
its volume of water, to which an equal volume of purified ether
is then added. The liquid is well cooled in a freezing mixture
and a cooled 33 per cent, solution of caustic soda is slowly added
until the liquid is just alkaline, and then an equal volume of a
saturated solution of potassium carbonate. The mass is now
well shaken and the ether decanted. In this way the ester,
which is rapidly hydrolysed by alkali at the ordinary tempera-
ture, is liberated from the hydrochloride without decomposition
and dissolves in the ether. The residue is kept in the freezing
mixture, a fresh quantity of ether, more caustic soda solution, and
sufficient solid potassium carbonate to form a pasty mass are
added in succession, shaken up thoroughly and the ether de-
canted. The residue is extracted two or three times with fresh
ether and the united extract, freed as far as possible from water,
is shaken up for a minute with solid potassium carbonate and
then dehydrated overnight with anhydrous sodium sulphate.
The ether is removed on the water-bath and the residue distilled
at a pressure not exceeding 15 mm. The colourless liquid, which
distils at 80100, has an ammoniacal smell and is nearly pure
leucine ester. Yield 1015 grams. The ester is readily
hydrolysed by boiling five times its weight of water with reflux
condenser until the alkaline reaction disappears (about an
hour). The liquid is then concentrated on the water-bath until
crystals separate on the surface and cooled. The leucine
may be recrystallised from dilute alcohol or dissolved in
the smallest quantity of hot water and alcohol added until
a turbidity appears. It forms small glistening plates, which
melt and sublime at 170. See Appendix, p. 270,